This invention relates to an electrical power amplifier device operating in a high frequency band as a saturation amplifier device.
In the conventional way an electrical power amplifier device employs DC bias supplying methods such as class A, class AB, class B and class C or the like. The class A is used for determining a bias point in such a way that a load line is always kept within a region where a current of a transistor flows and thus an output waveform when a sinusoidal wave is inputted in respect to the class A electrical power amplifier device designed in this way shows a sinusoidal voltage waveform and a sinusoidal current waveform, respectively. Class B is used for determining that a bias point is set to a threshold voltage of a transistor and an output waveform attained when a sinusoidal wave is inputted in respect to the class B electrical power amplifier device designed in this way becomes such a waveform as one in which only a half-cycle of positive sinusoidal wave is taken out. Comparing it with the class A shows that the class B has a high efficiency but has a poor linear characteristics.
The class AB is used for determining a bias point between the class A and the class B, and an output waveform when a sinusoidal wave is inputted to the class AB electrical power amplifier device designed in this way becomes a waveform in which a lower part of the sinusoidal wave is cut. An efficiency and a linear characteristic are set between the class A and the class B. The class C is used for determining a bias point to a value less than a threshold voltage, wherein an output waveform when a sinusoidal wave is inputted to the class C electrical power amplifier device designed in this way becomes such a waveform as one in which only a part near a top point of the sinusoidal wave is taken out. An efficiency is the highest value and a linear characteristic becomes the lowest one.
In addition, a gradient of the load line may influence against a gain and a saturation output when a low signal is inputted, wherein as a gradient of the load line is increased, the small signal gain is decreased and the saturation output is improved. To the contrary, as the gradient of the load line is decreased, the small signal gain is improved and the saturation output is decreased.
As an example of the prior art, a configuration of the electrical power amplifier device using a source grounded FET biased to the class AB is shown in FIG. 11. In addition, a load line of the same electrical power amplifier device is shown in FIG. 12. Further, both an output voltage waveform and an output current waveform of the same electrical power amplifier device are shown in FIG. 13. The output voltage amplitude in the case of class A has the maximum value at a value twice a voltage having a knee voltage V.sub.knee subtracted from the power supply voltage, and the amplitudes in the case of classes AB, B and C are less than this voltage. In this case, the knee voltage V knee is defined as a drain voltage when an increasing of the drain current is stopped.
In the case that it is tried to obtain a higher output electrical power with the same power supply voltage, the prior art employs two kinds of method, i.e. (i) a load impedance is decreased and an output current is increased, and (ii) a size of a transistor is increased and an output current is also increased.
However, as the problems of the prior art, it is possible to show the following problems:
(a) a gain is decreased when a load impedance is decreased; PA0 (b) a loss is increased since a current is increased; PA0 (c) as a gain is decreased and a loss is increased; an efficiency of added electrical power is deteriorated; and PA0 (d) an amplifier device is increased in size when a size of a transistor is increased.